1. Field of the Invention
The present invention relates to an alternating current power control device, and more especially to an alternating current power control circuit capable of controlling the amount of power energy supplied to an electric load by controlling cycle numbers of an alternating current power source in a predetermined cycle period.
2. Description of the Prior Art
In industrial applications, it is often necessary to control the power energy of an alternating current. For example, in controlling the rotary speed of an alternating current motor, various known approaches may be used to control the electric power energy supplied to the motor by using means, such as an inverter, a TRIAC, a mechanical gear, or by changing the winding turns of the motor.
In case that the rotary speed of the motor is required to be controlled precisely, a known inverter is frequently used in the prior art. However, the control circuit of the inverter commercially available in this art is relatively complex and the cost is very high. Thus, in some applications where precise control is unnecessary, the conventional inverter does not meet the requirements in economic consideration.
For example, in the rotary speed control of a conventional heat radiating fan, it is desired to control the rotary speed of the fan motor according to the temperature of environment so to save more power energy. That is, when the environment temperature is high, the rotary speed of the fan is increased to enhance the heat dissipating capability; while when the environment temperature is low, the rotary speed of the fan may be decreased to save power energy.
In order to control the rotary speed of the fan motor in prior art, a TRIAC is frequently used. Basically, the TRIAC controls the amount of power energy supplied to the load by controlling the phase angle of each cycle of the alternating current power source. However, the control circuit by using TRIAC will generate serious electromagnetic interference during turn on or turn off the current flow.